NAME

perlmod - Perl modules (packages and symbol tables)

DESCRIPTION

Packages

Perl provides a mechanism for alternative namespaces to protect packages from stomping on each other's variables. In fact, there's really no such thing as a global variable in Perl (although some identifiers default to the main package instead of the current one). The package statement declares the compilation unit as being in the given namespace. The scope of the package declaration is from the declaration itself through the end of the enclosing block, eval, sub, or end of file, whichever comes first (the same scope as the my() and local() operators). All further unqualified dynamic identifiers will be in this namespace. A package statement only affects dynamic variables--including those you've used local() on--but not lexical variables created with my(). Typically it would be the first declaration in a file to be included by the require or use operator. You can switch into a package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by prefixing the identifier with the package name and a double colon: $Package::Variable. If the package name is null, the main package is assumed. That is, $::sail is equivalent to $main::sail.

The old package delimiter was a single quote, but double colon is now the preferred delimiter, in part because it's more readable to humans, and in part because it's more readable to emacs macros. It also makes C++ programmers feel like they know what's going on--as opposed to using the single quote as separator, which was there to make Ada programmers feel like they knew what's going on. Because the old-fashioned syntax is still supported for backwards compatibility, if you try to use a string like "This is $owner's house", you'll be accessing $owner::s; that is, the $s variable in package owner, which is probably not what you meant. Use braces to disambiguate, as in "This is ${owner}'s house".

Packages may be nested inside other packages: $OUTER::INNER::var. This implies nothing about the order of name lookups, however. All symbols are either local to the current package, or must be fully qualified from the outer package name down. For instance, there is nowhere within package OUTER that $INNER::var refers to $OUTER::INNER::var. It would treat package INNER as a totally separate global package.

Only identifiers starting with letters (or underscore) are stored in a package's symbol table. All other symbols are kept in package main, including all of the punctuation variables like $_. In addition, when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC, and SIG are forced to be in package main, even when used for other purposes than their builtin one. Note also that, if you have a package called m, s, or y, then you can't use the qualified form of an identifier because it will be interpreted instead as a pattern match, a substitution, or a transliteration.

(Variables beginning with underscore used to be forced into package main, but we decided it was more useful for package writers to be able to use leading underscore to indicate private variables and method names. $_ is still global though.)

Eval()ed strings are compiled in the package in which the eval() was compiled. (Assignments to $SIG{}, however, assume the signal handler specified is in the main package. Qualify the signal handler name if you wish to have a signal handler in a package.) For an example, examine perldb.pl in the Perl library. It initially switches to the DB package so that the debugger doesn't interfere with variables in the script you are trying to debug. At various points, however, it temporarily switches back to the main package to evaluate various expressions in the context of the main package (or wherever you came from). See perldebug.

The special symbol __PACKAGE__ contains the current package, but cannot (easily) be used to construct variables.

See perlsub for other scoping issues related to my() and local(), and perlref regarding closures.

Symbol Tables

The symbol table for a package happens to be stored in the hash of that name with two colons appended. The main symbol table's name is thus %main::, or %:: for short. Likewise symbol table for the nested package mentioned earlier is named %OUTER::INNER::.

The value in each entry of the hash is what you are referring to when you use the *name typeglob notation. In fact, the following have the same effect, though the first is more efficient because it does the symbol table lookups at compile time:

local *main::foo = *main::bar;
local $main::{foo} = $main::{bar};

You can use this to print out all the variables in a package, for instance. The standard dumpvar.pl library and the CPAN module Devel::Symdump make use of this.

Assignment to a typeglob performs an aliasing operation, i.e.,

*dick = *richard;

causes variables, subroutines, formats, and file and directory handles accessible via the identifier richard also to be accessible via the identifier dick. If you want to alias only a particular variable or subroutine, you can assign a reference instead:

*dick = \$richard;

Which makes $richard and $dick the same variable, but leaves @richard and @dick as separate arrays. Tricky, eh?

This mechanism may be used to pass and return cheap references into or from subroutines if you won't want to copy the whole thing. It only works when assigning to dynamic variables, not lexicals.

On return, the reference will overwrite the hash slot in the symbol table specified by the *some_hash typeglob. This is a somewhat tricky way of passing around references cheaply when you won't want to have to remember to dereference variables explicitly.

Another use of symbol tables is for making "constant" scalars.

*PI = \3.14159265358979;

Now you cannot alter $PI, which is probably a good thing all in all. This isn't the same as a constant subroutine, which is subject to optimization at compile-time. This isn't. A constant subroutine is one prototyped to take no arguments and to return a constant expression. See perlsub for details on these. The use constant pragma is a convenient shorthand for these.

You can say *foo{PACKAGE} and *foo{NAME} to find out what name and package the *foo symbol table entry comes from. This may be useful in a subroutine that gets passed typeglobs as arguments:

The *foo{THING} notation can also be used to obtain references to the individual elements of *foo, see perlref.

Package Constructors and Destructors

There are two special subroutine definitions that function as package constructors and destructors. These are the BEGIN and END routines. The sub is optional for these routines.

A BEGIN subroutine is executed as soon as possible, that is, the moment it is completely defined, even before the rest of the containing file is parsed. You may have multiple BEGIN blocks within a file--they will execute in order of definition. Because a BEGIN block executes immediately, it can pull in definitions of subroutines and such from other files in time to be visible to the rest of the file. Once a BEGIN has run, it is immediately undefined and any code it used is returned to Perl's memory pool. This means you can't ever explicitly call a BEGIN.

An END subroutine is executed as late as possible, that is, when the interpreter is being exited, even if it is exiting as a result of a die() function. (But not if it's polymorphing into another program via exec, or being blown out of the water by a signal--you have to trap that yourself (if you can).) You may have multiple END blocks within a file--they will execute in reverse order of definition; that is: last in, first out (LIFO).

Inside an END subroutine, $? contains the value that the script is going to pass to exit(). You can modify $? to change the exit value of the script. Beware of changing $? by accident (e.g. by running something via system).

Note that when you use the -n and -p switches to Perl, BEGIN and END work just as they do in awk, as a degenerate case. As currently implemented (and subject to change, since its inconvenient at best), both BEGINandEND blocks are run when you use the -c switch for a compile-only syntax check, although your main code is not.

Perl Classes

There is no special class syntax in Perl, but a package may function as a class if it provides subroutines to act as methods. Such a package may also derive some of its methods from another class (package) by listing the other package name in its global @ISA array (which must be a package global, not a lexical).

Perl Modules

A module is just a package that is defined in a library file of the same name, and is designed to be reusable. It may do this by providing a mechanism for exporting some of its symbols into the symbol table of any package using it. Or it may function as a class definition and make its semantics available implicitly through method calls on the class and its objects, without explicit exportation of any symbols. Or it can do a little of both.

For example, to start a normal module called Some::Module, create a file called Some/Module.pm and start with this template:

Then go on to declare and use your variables in functions without any qualifications. See Exporter and the perlmodlib for details on mechanics and style issues in module creation.

Perl modules are included into your program by saying

use Module;

or

use Module LIST;

This is exactly equivalent to

BEGIN { require Module; import Module; }

or

BEGIN { require Module; import Module LIST; }

As a special case

use Module ();

is exactly equivalent to

BEGIN { require Module; }

All Perl module files have the extension .pm. use assumes this so that you don't have to spell out "Module.pm" in quotes. This also helps to differentiate new modules from old .pl and .ph files. Module names are also capitalized unless they're functioning as pragmas, "Pragmas" are in effect compiler directives, and are sometimes called "pragmatic modules" (or even "pragmata" if you're a classicist).

The two statements:

require SomeModule;
require "SomeModule.pm";

differ from each other in two ways. In the first case, any double colons in the module name, such as Some::Module, are translated into your system's directory separator, usually "/". The second case does not, and would have to be specified literally. The other difference is that seeing the first require clues in the compiler that uses of indirect object notation involving "SomeModule", as in $ob = purge SomeModule, are method calls, not function calls. (Yes, this really can make a difference.)

Because the use statement implies a BEGIN block, the importation of semantics happens at the moment the use statement is compiled, before the rest of the file is compiled. This is how it is able to function as a pragma mechanism, and also how modules are able to declare subroutines that are then visible as list operators for the rest of the current file. This will not work if you use require instead of use. With require you can get into this problem:

In general, use Module () is recommended over require Module, because it determines module availability at compile time, not in the middle of your program's execution. An exception would be if two modules each tried to use each other, and each also called a function from that other module. In that case, it's easy to use requires instead.

Perl packages may be nested inside other package names, so we can have package names containing ::. But if we used that package name directly as a filename it would makes for unwieldy or impossible filenames on some systems. Therefore, if a module's name is, say, Text::Soundex, then its definition is actually found in the library file Text/Soundex.pm.

Perl modules always have a .pm file, but there may also be dynamically linked executables or autoloaded subroutine definitions associated with the module. If so, these will be entirely transparent to the user of the module. It is the responsibility of the .pm file to load (or arrange to autoload) any additional functionality. The POSIX module happens to do both dynamic loading and autoloading, but the user can say just use POSIX to get it all.

SEE ALSO

See perlmodlib for general style issues related to building Perl modules and classes as well as descriptions of the standard library and CPAN, Exporter for how Perl's standard import/export mechanism works, perltoot for an in-depth tutorial on creating classes, perlobj for a hard-core reference document on objects, and perlsub for an explanation of functions and scoping.